Experiments are proposed to investigate some of the physiological changes that take place in the peripheral auditory system as the result of aging in a quiet environment and aging in an environment of low-level noise. Our major goal is to identify some of the mechanisms in the cochlea that deteriorate with age and chronic noise exposure. Preliminary results suggest that dysfunction in the ion-pumping tissue along the cochlear duct may be responsible for certain types of hearing loss seen with age. Studies are also proposed to examine some effects of age and noise on the second-order neurons in the cochlear nucleus. The purpose of the cochlear nucleus studies is to begin to examine how the central nervous system responds to long-term low-level exposures of noise. The proposed work has direct bearing on human studies concerned with the gradual loss of hearing often found in the elderly, a process termed presbyacusis. Systematic studies in animals under controlled conditions will provide an opportunity to test various hypotheses of how age and environment interact to change the peripheral auditory system.